Abstract: This paper presents a simulation study on multiple voltage rectification in a high-voltage frequency converter system and obtains the DC-side waveform after rectification. Analysis shows that multiple voltage rectification has a good suppression effect on DC voltage pulsation in the high-voltage frequency converter system.
Keywords: high voltage frequency converter, multiple voltages, DC pulsation
Abstract: In this paper, the multiply voltage rectification of high-voltage frequency converter will be analyzed by emluator. And then the DC side wave of rectifier can be got. After comparing and analyzing these waves, the restrainable effect of multiply voltage rectification to DC pulsant of high-voltage frequency converter will be very clear.
Key words: high-voltage frequency converter; multiply voltage; DC pulsant
1. Introduction
In the steel industry, the transmission system is the core equipment; the direct actuator for transmission implementation and control is the electric motor, and motor control is arguably a major aspect of the steel industry. In many fields, especially high-voltage and high-power applications, DC motors have been phased out. Although DC motors offer excellent speed regulation, their inherent structural weaknesses are difficult to overcome. Currently, AC motors dominate the transmission field, especially with the development of frequency converter technology, which has further enhanced their advantages. However, in the high-voltage applications frequently encountered in the steel industry, AC frequency converter technology is not yet fully perfect, and there are still some areas worthy of discussion among technicians. This article will analyze the input rectification section of high-voltage frequency converters to examine the characteristics of the pulsating voltage on the DC side of the frequency converter under multiple voltage rectification.
2. Multi-voltage input structure of high-voltage frequency converter
2.1 Series Structure of Power Units in High-Voltage Frequency Converters
In my country, high-voltage frequency converters are defined as those with a high-voltage input of 6000V or higher. In industrial practice, 6000V and 10000V high-voltage inputs are most commonly used. Under such high voltages, the traditional circuit structure of low-voltage frequency converters cannot be used directly because the power unit devices cannot withstand such high voltage levels. Therefore, to meet the requirements, a series connection method of power units is used in practice, allowing low-voltage devices to be connected in series to divide the voltage and achieve the purpose of high-voltage output.
It is important to emphasize that the series connection of power units differs fundamentally from the series connection of power devices. While series connection of power devices can solve the problem of insufficient voltage withstand capability of a single power device, it is difficult to achieve voltage equalization among the connected devices. Uneven voltage equalization results in the power devices being damaged by breakdown under high voltage. In contrast, the series connection of power units involves breaking down a high-voltage input into several low-voltage units, which are then connected in series on the output side to form a high-voltage output. Therefore, the voltage equalization problem does not exist. Figure 2.1 shows the structure of the series connection of power units. This paper focuses on the rectification of the input, so only the structural principle of the rectifier side is described.
Figure 2.1 Structural principle of power units connected in series
2.2 Analysis of the Multiplexing Principle of Rectified Input Voltage
One crucial application of transformers is phase transformation. To achieve multi-pulse rectification, especially with input multiplexing, it's desirable to obtain multiple lead or lag angles so that different series units are at different electrical angles. This serves two important purposes: firstly, it minimizes DC-side voltage ripple after rectification; secondly, it reduces harmonic interference on the power supply side. Harmonic reduction is crucial for green and intelligent industrial power supply, ensuring power quality and minimizing interference between equipment.
In industrial power systems, the incoming line of a high-voltage frequency converter is high voltage. By connecting power units in series, each power unit can process low voltage, which is then connected in series at the output side to form high voltage to achieve the desired effect. The high voltage of the incoming line needs to be transformed into a low voltage that the devices can withstand by a transformer, while also ensuring minimal harmonic interference. Therefore, it is necessary to fully utilize the transformer's functions of voltage transformation, current transformation, and phase transformation, so that the inputs of different units differ by an electrical angle, achieving the purpose of multiple voltage rectification.
In practical applications, the extended triangle method is commonly used for electrical angle phase shifting. The electrical derivation of the extended triangle method is detailed in many sources and will not be repeated here; only a few conclusions and formulas are given. A diagram of the extended triangle phase shift is shown in Figure 2.2.
Figure 2.2 Structural principle of extended triangle phase shift
The following two are important conclusions and formulas: Here, θ is the phase angle by which the secondary line voltage leads the primary phase voltage. Let the number of turns of the primary and secondary windings of the transformer be N1 and N2 , respectively, and the turns ratio be n. The number of turns of the phase-shifting winding and the basic winding on the secondary side are kN2 and (1- kN2 ), respectively. Of course, k is a positive number between 0 and 1.
It can be clearly seen that when k is between 0 and 1, this extended delta configuration can achieve a phase lag shift of α from -π/6 to 0 degrees. If a lag of 3.75° is required and the primary input voltage is 6000V, then k = 0.796 and n = 0.0741 can be calculated. This overcomes the limitation of traditional star-delta connections where transformers can only shift phase in multiples of 30°.
3. Multiplexed waveform analysis of rectified input voltage
Theoretically, multiple rectification has two advantages: first, it reduces voltage ripple on the DC bus, improving the quality of the DC power supply, which is beneficial for inverters; second, it reduces harmonic interference on the power supply side, minimizing the impact on the power grid and other equipment, especially precision equipment. This paper focuses on analyzing the voltage ripple problem on the DC bus, without delving into extensive theoretical derivations and calculations. Instead, it directly utilizes computer simulations to indirectly demonstrate the significance of multiple voltage rectification and verify its theoretical conclusions.
In the Matlab environment, a model is used to simulate the waveform after rectification, and a comparison is made using 4-unit series connection and 8-unit series connection as examples. The waveforms are shown in Figures 3.1 and 3.2:
Figure 3.1 DC-side voltage waveform of four units connected in series
Figure 3.1 shows the case of four power units connected in series with a phase shift of 15° between groups. It can be seen that although the waveform appears to fluctuate greatly, it actually only fluctuates within a range of about 20V. Compared to the output of several kilovolts, the voltage is very stable.
Figure 3.2 DC-side voltage waveform of eight units connected in series
Figure 3.2 shows the simulated waveforms of eight power units connected in series. The highest output voltage in the figure is 6798V, while the lowest voltage is around 6795V, a difference of less than 3V. This significantly improves the stability of the DC voltage. It is evident that the DC voltage output from the multi-pulse rectifier has relatively high quality.
Figures 3.3 and 3.4 are enlarged views of the DC side voltage of the eight-unit series connection.
Figure 3.3 Partial enlarged view of the output DC voltage of the eight-unit series connection 1
Figure 3.4 Partial enlarged view of the output DC voltage of the eight-unit series connection.
The conclusion that can be drawn from the above four figures is that the DC voltage ripple after the power units are connected in series is very small and can be almost ignored. As an intermediate transition link of the frequency converter, especially in high-voltage and high-power frequency converter systems, the stability of the DC power supply is of great significance to the high-quality output of the inverter side.
In the dust removal fans of the blast furnaces at Wuhan Iron and Steel Plant, multiple voltage input rectification technology is widely used, and its effect is very ideal. It not only meets the process requirements, but also achieves the purpose of energy saving and environmental protection, and has a relatively small impact on the power grid. Compared with traditional control methods, its effect is obvious.
4. References
[1] Zhang Hao, Xu Mingjin, Yang Mei, eds. High Voltage High Power AC Variable Frequency Speed Regulation Technology. Beijing: Machinery Industry Press, 2006.
[2] Yao Lan, Liu Wenzhong. Application and precautions of high voltage frequency converter in blower and suction fan. Hebei Electric Power Technology, 2005, No. 3
[3] Lin Weixun. Modern Power Electronics Technology [M]. Beijing: Machinery Industry Press, 2008.
